Gallium nitride crystal substrates are widely used as substrates of a variety of semiconductor devices. Improvement of properties of semiconductor devices requires low dislocation density GaN wafers.
(1) Japanese Patent Laying Open No. 2003-165799 and (2) Japanese Patent Laying Open No. 2003-183100 newly proposed facet-growth methods for making low-dislocation density GaN crystal wafers. These are named a facet-growth method by the Inventors. The facet growth method reduces dislocations by making masks on an undersubstrate, growing gallium nitride on the masked undersubstrate, building facets at predetermined spots, combining dislocations of different signs at the facets and annihilating dislocations.
The above facet growth method can decrease dislocations and can make low-dislocation GaN substrate wafers. Bottom parts, which had been nearer to the undersubstrate, have higher dislocation densities. Top parts, which had been farther distanced from the undersubstrate, have lower dislocation densities. There is a gradient of dislocation density in the direction of thickness in the face-growth-method-made GaN wafers. The dislocation density gradient in the vertical direction induces large distortion in GaN wafers, which is equivalent to a small curvature radius. Large distortion prevents the GaN wafers from growing uniform epitaxial layers thereon. Ununiformity of epitaxial layers prohibits the GaN wafers from making semiconductor devices having uniform properties.
A purpose of the present invention is to provide a low-distortion gallium nitride substrate and a method of producing a low-distortion gallium nitride substrate. Low distortion means small bending of a surface. Low or small distortion is equivalent to a long or wide curvature radius, since a curvature radius is a reciprocal of a curvature.